Engine ignition systems may include a spark plug for delivering an electric current to a combustion chamber of a spark-ignited engine to ignite an air-fuel mixture and initiate combustion. Based on engine operating conditions, spark plug fouling can occur wherein a firing tip of the spark plug insulator becomes coated with a foreign substance, such as fuel, oil, or soot. For example, vehicles may be moved over short distances multiple times, such as to various temporary storage locations in an assembly plant during vehicle assembly, with the engine running for short periods of time. These short engine cycles can lead to carbon and other deposits forming on the spark plugs, resulting in spark plug fouling and undesirable engine performance. The problem may be exacerbated in engines running with direct fuel injection due to increased spray impingement of fuel on the spark plug from the direct fuel injector.
Once fouled, the spark plug may be unable to provide adequate voltage to trigger cylinder combustion until the spark plug is sufficiently cleaned or replaced. The spark plug may be cleaned by burning off the soot accumulated on the fouled spark plug by operating the engine in speed-load conditions that sufficiently raise the spark plug tip temperature. However, the short vehicle trips at the assembly plant do not allow for adequate heat transfer to burn off the soot.
Various strategies have been developed to address spark plug fouling associated with short run times at the assembly plant. One example approach shown by Glugla et al. in U.S. Pat. No. 8,397,695 employs an alternate engine calibration with progressively more aggressive control procedures to develop more heat in the combustion chambers and eliminate any spark plug deposits. The control procedures may include repetitive sparking, exhaust stroke sparking, increased engine loading, advanced spark timing, lean air/fuel ratio, and elevated engine idle speed. Still other approaches for reducing fouling including disabling exhaust gas recirculation and limiting an engine power output.
However, the inventors herein have identified potential issues with such approaches. As an example, the alternate calibration may be employed only for a limited number of engine starts and/or may expire after a defined mileage is driven in a given trip. Alternatively, the calibration may be disabled before the vehicle is delivered to a customer. Thereafter, the engine controller may resume a default production calibration, and the alternate calibration may not be accessed again. However, the spark plugs may remain fouled even after the alternate calibration is in effect, degrading engine performance after vehicle delivery. Furthermore, the lean air/fuel ratio of the alternate calibration may result in engine stalling, particularly during cold starts, and the higher engine idle speed may be objectionable to some customers. As another example, some customers may operate the vehicle under similar conditions with short drive cycles that facilitate spark plug deposit formation. In such cases, the spark plugs may continue to be prone to spark plug fouling. The elevated propensity for spark plug fouling can result in low mile warranty issues.
In one example, the issues described above may be addressed by a method for an engine comprising: responsive to an engine shutdown request, after disabling cylinder fueling, intermittently discharging an ignition coil of a spark plug, discharge parameters of the discharging based on a soot load of the spark plug. In this way, spark plug fouling can be addressed more effectively.
As one example, while a vehicle is being assembled and/or operated at an assembly plant, the engine may be operated with a first adjusted ignition coil discharge schedule (see FIG. 6, schedule B). The first discharge schedule may be enabled responsive to the vehicle being at the assembly plant and the engine being in a green engine condition. Furthermore, the first discharge schedule may be enabled before an indication of spark plug fouling is received so as to pre-empt spark plug fouling. Therein, a spark plug ignition coil may be discharged a plurality of times during engine spin-down and while the engine is at rest in order to raise spark plug tip temperatures, and burn away any accumulated soot. In addition, the spark plug ignition coil may be discharged before the engine is started and before the vehicle is driven. A discharging frequency as well as a discharge time on each discharge event may be adjusted to be higher in the first schedule, the discharge parameters based on the soot load (actual or expected) of the spark plug during the green engine condition. The opportunistic discharging enables spark plug fouling issues in a green engine (where the vehicle is still at the assembly plant) to be reduced. In comparison, when the vehicle is operated after leaving the assembly plant, the engine may be recalibrated to be operated with a second, different ignition coil discharge schedule (see FIG. 6, schedule D). The second schedule may be selectively enabled responsive to an indication of spark plug fouling at the non-green engine. Therein, the spark plug ignition coil may be discharged a plurality of times during engine spin-down and at rest, as well as before an engine start preceding a vehicle launch. The second schedule may include a lower duty cycle and a shorter discharge time as compared to the first schedule, the discharge parameters based on the soot load (actual or expected) of the spark plug during the non-green engine conditions. Once it is determined that the spark plug is sufficiently clean, or sufficiently warm, ignition coil discharge during engine spin-down and before engine restart may be disabled.
In this way, spark plug fouling issues can be reduced. The technical effect of frequently discharging the ignition coil during engine spin-down and after the engine is at rest is that spark plug tip temperatures may be raised to, and held at, higher temperatures for a longer duration, improving spark plug health. Further, spark plug cleaning can be performed without compromising emission standards, fuel economy, and engine performance benefits. By better addressing spark plug fouling in a green engine, low mile warranty issues are reduced. By improving spark plug health, cylinder misfire events can be reduced and, engine component life may be extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.